Induction motors are widely employed for a variety of applications and are available in a wide range of capacities, from fractional horsepower to multiple horsepower sizes. The speed of such motors cannot be efficiently reduced by lowering the applied voltage. This is because, when the speed is reduced, the stator and rotor currents substantially increase and the power factor substantially decreases. This results in a significant loss in motor efficiency, with damage to the motor possible by reason of the increased currents and consequent increased temperature due to increased ohmic losses. The increase in ohmic losses in a conventional induction motor, occasioned by reduced input voltage and thus reduced speed, can become so large as to render continuous operation impractical. Therefore, other methods have been developed to vary the speed of an induction motor.
One such conventional method is shown in FIG. 1. In FIG. 1, the speed of an induction motor 10 is controlled by an adjustable speed drive (ASD) 20 which includes an ac to dc rectifier 22, voltage regulator 24 and filter 26 which supply dc power to a dc to ac converter 28. The voltage and frequency output of the dc to ac converter 28 is controlled by a frequency controller 30 and drives the motor 10 at a speed related to the voltage and frequency of output of the dc to ac converter 28. This conventional adjustable speed drive (ASD) 20 not only requires electronic circuitry for providing the ac to dc rectification, filtering and the dc to ac conversion, but also for voltage regulation and frequency control.
A drawback of such an ASD system is that it typically generates harmonic currents which are not desirable and which can adversely affect the ac line supply. Further, such ASD systems are also sensitive to transients and surge voltages which can exist on the ac power supply line.
It is therefore advantageous to provide means for controlling the speed of an induction motor without the need for the relatively complex and sensitive electronics for varying the applied voltage and frequency.